Support serialization of the compiler backbone.

pkg/compiler/lib/src/constants/expressions.dart

Index: pkg/compiler/lib/src/constants/expressions.dart
diff --git a/pkg/compiler/lib/src/constants/expressions.dart b/pkg/compiler/lib/src/constants/expressions.dart
index 5644b0f5847ce5db238ce8f803a226a1c2904470..abb3c6dec439f7884e1d0fd86a94b3a3c4115a10 100644
--- a/pkg/compiler/lib/src/constants/expressions.dart
+++ b/pkg/compiler/lib/src/constants/expressions.dart
@@ -107,6 +107,22 @@ abstract class ConstantConstructor {
   Map<FieldElement, ConstantExpression> computeInstanceFields(
       List<ConstantExpression> arguments,
       CallStructure callStructure);
+
+  accept(ConstantConstructorVisitor visitor, arg);
+}
+
+abstract class ConstantConstructorVisitor<R, A> {
+  const ConstantConstructorVisitor();
+
+  R visit(ConstantConstructor constantConstructor, A context) {
+    return constantConstructor.accept(this, context);
+  }
+
+  R visitGenerative(GenerativeConstantConstructor constructor, A arg);
+  R visitRedirectingGenerative(
+      RedirectingGenerativeConstantConstructor constructor, A arg);
+  R visitRedirectingFactory(
+      RedirectingFactoryConstantConstructor constructor, A arg);
 }
 
 /// A generative constant constructor.
@@ -141,6 +157,10 @@ class GenerativeConstantConstructor implements ConstantConstructor{
     return appliedFieldMap;
   }
 
+  accept(ConstantConstructorVisitor visitor, arg) {
+    return visitor.visitGenerative(this, arg);
+  }
+
   String toString() {
     StringBuffer sb = new StringBuffer();
     sb.write("{'type': $type");
@@ -205,6 +225,10 @@ class RedirectingGenerativeConstantConstructor implements ConstantConstructor {
     return appliedFieldMap;
   }
 
+  accept(ConstantConstructorVisitor visitor, arg) {
+    return visitor.visitRedirectingGenerative(this, arg);
+  }
+
   String toString() {
     StringBuffer sb = new StringBuffer();
     sb.write("{'type': ${thisConstructorInvocation.type}");
@@ -240,6 +264,10 @@ class RedirectingFactoryConstantConstructor implements ConstantConstructor {
     return constantConstructor.computeInstanceFields(arguments, callStructure);
   }
 
+  accept(ConstantConstructorVisitor visitor, arg) {
+    return visitor.visitRedirectingFactory(this, arg);
+  }
+
   String toString() {
     StringBuffer sb = new StringBuffer();
     sb.write("{");